Personally I think the Leif house can be a bit tight for a Syrian - the entrance holes are only about 5cm diameter which might be ok for a smaller syrian. You might be better with a larger open house. The Ferplast Guinea pig house is a good size for a hamster - however not big enough to fit any kind of litter tray inside as well.
Before I had the Rodipet labyrinth house I actually had the Ferplast Sin Rabbit house - which worked great. It was quite tall so had it stood on the base of the cage with substrate piled up round the outside and some inside. This meant it was stable and couldn't sink (not possible to tunnel under) and substrate inside reduced the effective height.
And it was plenty big enough to put a litter tray in. It should be fine in a 100cm by 50cm cage. I had it in an 80 x 50 cage and it took up far too much space so upgraded to a 100 x 50 ish.
It fitted well at one end so the width of the house (the shorter side) was facing the longest part of the cage and the length of the house fitted into the 50cm width.
I think it's actually about the same size - external dimensions - as the Leif house - maybe a bit shorter.
https://www.amazon.co.uk/Ferplast-46...gateway&sr=8-1
Maybe your best bet would be a shoebox house for now. Similar idea to the rabbit house. Cut the base out of a shoebox so the house is sat on the substrate and open underneath, cut a door at one end of the long side (so the other end is dark inside) and keep the lid as a lift off roof.