You can easily use a DIY method, pick a large shallow baking tray large enough for your bum to fit into. Mix some flour, eggs, butter and sugar together enough to fill about halfway up the pan, pour this into the pan and bake at gas mark 5 for 20 minutes, take out and allow to cool.
Place pan with slightly cooled contents on a chair, remove keks and lower your bottom onto the now cooked mix, stay still for a few minutes without moving, then gently raise yourself, hey presto you have a mould to take your measurements from.
If on standing some of the mix comes away with you, then I suggest carrying out the whole proceedings again from the start, this time greasing your bum.
The wife would not let me use one of her pans, so I went to a local bike shop, not a lot of use to you though as Lincoln is not local to you.
Must say I was pleased with the Botrager I bought after being measured.