Well, it will be an interesting project Kevin, tho, 1st things 1st I reckon, get it built then if there's a will, theres a way - even Re: Geckos !
Well, that's my intitial bob's worth
Tho thunking, if was my issue, initially need gaps very small. On my Caboose I have rubber mat cut into strips, tacked on the bottom edge, just rubbing against the ground 'crete - nothing much getting in at all, tho I do spay now and then Surface Bug spray to stop ants, and that works until a good days rain
Might be able to apply similar between your walls and roof ?
Also, the gecko's hang around the house, speshly near windows, they wait there till the light comes on, our kitchen light is on and off during the night, every night, which in turn attracts bugs, moths, and the geckos are there in waiting for their nightly feed !
Strategy for the Obs, maybe place some solar lights away from Obs and they should continually hang around them ! Also, the Obs does not have bright white Lights, so no attraction for bugs, hence geckos hanging around other light sources
Some ideas to ponder on, any which, why, how.