My leopards have a totally flat plastron and both sexed as females, male concave is usually really noticeable. I never understand the comments referring they usually can't be sexed till 6 or 7 years old, it's pretty obvious after 18 months .
In a 13x3 he will be very restless. My leopards now live outdoors 24/7 with a heated 6x6 shed and a 60x10ft run and they stomp that all day pretty much non stop. They're a little bit bigger then him but not much, one is 10" and the other is 7", with all the grazing they can get.
If I were you I'd try to catch the back end of summer and grow some grasses/hay quickly. My leopards were on a grass/weed diet and it was good and convenient, but they're totally different animals on a mostly grass diet now. The growth is so much quicker and smoother. My oldest put on nearly a kilo in 6 months recently just through grazing.
We bought a bag of mixed grass seeds on eBay and noticed the buffalo grass grows in anything, winter or red hot summer so we littered the run last year and it just about survives with their grazing.
Good idea on the hot box section. We used some polycarbonate and some hinges with a thermostatic prop from a greenhouse to help regulate a hot box for ours, on the coldest of days aslong as the sun's out it'll get to 22* in there, then on the hottest days the thermo prop opens up and let's the warm air flow out so never gets hotter then 25*ish.
I'd love a sulcata but I don't think I could cope with a 3ft long tortoise tearing round my garden tunnelling haha.