My Christmas gift to my nephews was a tunnel for their garden train. Here are the steps I took.
1. measure the train to allow the right amount of clearance through the openings. i added about 2 inches to both measurements and that became the size of my openings which were 6 x 10ins. I then decided to make the 2 faces 14 x14 which gives me 4 ins on the top and either side of the opening. the opening is cut starting on the bottom edge as shown in the sketches and finished product below.
2. sketch out your design. i searched for images of real tunnels in order to get ideas. i got lucky and was given some tiny tiles left over from a bathroom remodel that was going on at the same time i was creating this. they worked perfect! i factored them into my design and figured i'd find some rocks some where to finish off the faces.
3. collecting supplies. you might have enough supplies lying around home to complete this project. the wood i had was not quite the right size and instead of spending more time and money piecing together smaller odd shaped pieces to get the size i wanted, i just bought a new piece of wood 5/8in thick and had most of the pieces cut down to size at home depot. (3 pieces 14in x 3ft.) the hardware was the most expensive part. i wanted to make sure the sides were sufficiently supported since this tunnel would some day have a pile of dirt on top of it to simulate a tunnel through a mountain. i bought a dozen wide L brackets (and a 50 pack of screws) and used 4 brackets to connect each face to the sides, 4 to connect the faces to the top, and 4 to connect the top to the sides.. i had some silicone and outdoor paint already. as i mentioned before, there was a bathroom remodel going on in the house where we were staying at the time, so i also got my hands on some of the mastic (its the glue that is used behind the tiles that holds them to the wall. it looks like cement). and for the last supply needed...the rocks...nick and i got up super early one december morning and went down to the arkansas river to collect rocks. we ended up with soooo many that i could do half a dozen more.
4. cut the openings. i used the tools that were available to me. i only own a circular saw so i used that to finish cutting the pieces i needed to make the tunnel. 2 14in x14in pieces for the faces. i also ended up shorting the 2 side pieces so that the faces sat under the top and in front of the sides. i took the width of the wood (5/8in) times 2 and then cut that amount off each of the side pieces. i then drew a 6x10in rectangle on each face then free-handed an arched top. but, i ended up making a template of half of the arch and then mirroring it on the other side so it was more uniform. i started the arch about half an inch from the top of the rectangle and the top of the arch was probably about half to 3/4in above the original rectangle. i cut the straight part with my circular saw and borrowed a jig saw for the arches.
5. assemble. i laid the top (bottom side up) on the ground and attached one of the faces first then the 2 sides and finally the last face.
6. decorate the faces. this is the part that makes your tunnel unique. set the box on one face so that the other face is level (like the top of a table). i applied the mastic around the arch first and set in the tiny tiles. i traced the face and opening on a piece of paper and spent some time arranging the rocks in a variety of sizes and colors. then i applied the mastic to the rest of the face and transfered the rocks one-by-one over to its place on the face. (someone a little more daring might just apply the mastic and start pulling rocks from a pile and applying them freely on the face. but im not that daring.) let the mastic fulling dry. if you have time, id say 24 hrs. if you dont have time (which i didnt), 12 hrs should be ok. flip the box over and do the same for the other face.
7. paint the box. this could also have been done before decorating the face. you could seal the would in another way i suppose but painting it is a quick way to do so. i used outdoor paint and gave it 2-3coats. make sure to get the bottom edges that would be sitting on the ground because they are the most likely to soak up the moisture from the ground.
7. seal the joints. i used silicone on the inside joints to keep the moisture out and add a little more stability. for some reason, the silicone i had was bad. it was super running and caused the caulk to pool in the corners. make sure to allow enough time for it to dry. depending on the caulk, it could take 48-72hrs to dry fully.