I've built a lean-to greenhouse 1.80metre long, 1.0 metre wide, about 3 metre high. In the floor I want to put a heat sink using an old polypropylene water butt (100 litre), one of those slim ones for narrow spaces. It's split at the bottom a bit so is now useless for water. Does anyone think, given the size of the greenhouse that it will be worthwhile or not using this size butt? Plan to fill it with broken glass. Been saving beer and wine bottles. There's no room for a bigger container for the glass. Thanks.