I don't what constitutes huge, but the GR/Rythmik dipole, when built as an H-frame (see below) and positioned standing up, is only 16" wide, 14" deep, and about 28" tall. The cool thing is the sub, being dipole, can be laid on it's side and used as a stand for the Quads---just like the sub Gradient made for the Quad 63. So positioned, the dimensions are now 16" tall (a good height for Quads) and about 28" wide, good for the old ("57"s) or new Quads.
The OB/Dipole sub is available as a kit only, consisting of two 12" woofers and the Rythmik plate amp, with a special dipole-cancellation compensation network/shelving circuit installed. The kit is installed in one of two OB "frames"---the "W" or the "H", both pictured on the GR Research website (in the pages detailing each version of the GR 12" free-air woofer). You can print out the diagram of either or both frames, and have a cabinet maker build you a pair. GR Research has a woodworker in Canada making them as a flat pack, but shipping to the UK might be prohibitive.
For a look at the finished product, see the beautiful virtual system of Audiogon member kennythekey. He has a pair of the subs in H-frames the Canadian builder veneered in wood to match Kenny's speakers. He is very happy with the subs.