Makita 9911 is a lot cheaper than the 9404 and can be clamped inverted. It needs clamps like these:
but they need to have 8mm bars (frustratingly not 8.5mm!). The proper things are on Makita UK's web site, and might be had special order, but won't be cheap.
I'm tempted to take a punt on the UJK ones ("Fence Clamps", from Axi.), but to be honest, a soft faced clamp on the front handle and a cable tie for the back one serve just as well if you're not being stupid with it.
None of them will DX well used upside down on the bench. they need the flat surface of the work underneath them, as
the dust is sucked off the back roller. If you want to use it inverted you'll need to rig something else up.
Two generic things: I have learned the hard way not to buy cheap belts.The proper ones cut better, last longer, and are more flexible (better finish and less damage to the sander). And stopping frequently to clean the belt with a crepe block makes a huge difference too.
E.
PS: If you really must get cheap belts, they usually have no lubrication and will wear the pad on the sander very fast and make it run hot. Rub the back of the belt liberally with a 4B or 6B pencil to add graphite (the Makita belts are graphite impregnated - another plus).