I have fooled with connex boxes and they can really be nice. To start we had three, 3-20 and 1-40 foot one. These are not buried but above ground with solar power and a back up generator when it is so overcast the solar is blocked.

The big thing initially was to clear away any rust and seal it. The exterior was primed with two coats and then two overcoats. If you were to buy one make sure you look at all the places which water would settle, hence rust. I did a smell test, all of them had wooden decks. I put down a vapor barrier, Tyvex, and then put 1 inch tongue and groove flooring down. Then I framed out the sides and ceiling. The sides were 2x4 and the ceiling was 2x6. I insulated the sides with two inch rigid foam and four inch ceiling. On the sides and ceiling I covered them in 1/2 inch plywood after wiring it for lighting and electrical outlets.

So what had this to do with a buried connex box? Well if I was going to bury one I would do it the same way but instead of wood I would use steel in place of the wood studs. On the floor I would use steel to tie in the walls to give it more rigidity and then lay the flooring on top. Over the exterior of the connex before I buried it I would cover it with butyl rubber material which is used in ponds. This covering would cover all sides and ends. Before I placed the connex I would lay down a good rock base with pipe underneath to drain any water which could accumulate under the connex. It would also need a ventilation system which draw exterior air both powered and manually. There are several good systems on the market.

Now for plan B. I have also looked at using culverts too and personally I think the 12 foot wide ones are the only ones viable. I agree they are stronger and would give you storage underneath and plenty of room up top. The thing I do not like about the culverts is the round tube, although it may be stronger I get a closed in feeling when I am in them. I prefer the box over the tube.

Cost wise I do not know what is cheaper. I do know a 20 foot box goes for about $2,500 plus shipping..