Just pop the seat & have a look to see if the space that was used for the battery in earlier models is still there?! If you open the LHS pannier & look in at the back of the space, the top front of Spyder space will have a battery sized 'box' impinging on the pannier storage space - you get at that void space by looking under the seat! There's likely a flap of the black placcy under-seat trim covering the top of the void, possibly even a wiring loom joiner or two in there taking up some of the space, but it's basically a 'battery sized void space' under the seat. This space is partially open to the under guard area above the back tire, so it can get dirty &/or wet if you are riding in those conditions, but once you've moved the wiring out of the way (just push it toward the front or back of the Spyder) it leaves about an 8"x4"x6-7" space that will hold a package or box about that size.... While the black placcy 'top' or cover for this hole isn't readily pushed open or shut, you could fairly easily score along the offside edge with a sharp knife to open up the cover, then fit a handle or knob on one side of the bit removed & a length of piano hinge or something to hinge the 'lid' on the cut side - and there you have it, a secure space to drop a tin or package into!
If you don't like the 'partially open to the under-guard tire space' bit, you can fairly easily find a cardboard box of the correct size or maybe even make a placcy box to drop into that hole. But once you've got access to the hole & protection from the elements & rubbish chucked around under the guard by the wheel sorted, you only hafta coil the spare belt up & wrap it in shrink wrap or even kitchen cling wrap before hiding it in that space. Now that I've mentioned 'hiding', it doesn't take a lot of imagination to see the advantage of having a fairly unobtrusive & relatively secure spot somewhere on your Spyder to hide a few important trinkets or papers - be careful about how you set up the 'lid' on this space & it can become an easily hidden custom made 'safe deposit box' under the seat... otherwise you can carry a spare belt in there. :thumbup:
Over to you