do you want round or square?? i sell loads of the all sizes all shapes
No seriously all you need to do is go to your local electrical wholesale outlet i.e newey & eyre, kew,tlc, city electrical, eyre & elliston or other and ask for a
IP65 box they are totally waterproof if you want to put a 3 way splitter inside you probably want something like a Gewiss box No GW44002 or GW44003
Gewiss = brand
IP65 = Means it's totally waterproof
by the way those two boxes should cost about £1.50 - £2.00 each
hope that helps
By the way the boxes you where looking at in maplins are only IP56 and are not totally waterproof (believe me there only water resistant)