This article refers to the fitting of a 'Temporary Use' spare tyre (T155/80R17), using an Australian Ford Falcon (Model FG) 'Temporary Use' wheel, under the rear floor of a 2021 Hyundai Kona EV.   Given that EVs often come without a spare, the concept might be adapted to other vehicles.
I should thank John Storey for his informative article documenting the fitting of a slightly smaller width (T135/80D17) tyre, in his Kona EV. He also used an Australian Ford Falcon (Model FG) 'Temporary Use' wheel.

The 'Tyre Mobility Kit' supplied with the Kona EV is essentially designed to temporarily repair a small puncture in the tread area of the tyre. A large puncture, a blowout, or damage to the side wall will probably require tyre replacement. In such a situation, the small 12 volt compressor and can of liquid sealant supplied with the car will be useless.
In addition, the Kona EV is fitted with a Tyre Pressure Monitoring System (TPMS). 'Single-use' sealant may not only render the tyre irreparable, but also foul the TPMS.  Some tyre dealers may decline to fix a tyre that has been repaired using temporary sealant.

Although a Kona EV full-size spare wheel (T215/55R17), would be ideal, it will definitely not fit under the rear floor. Enquiries to tyre dealers in Australia indicate that a new T135/80D17 tyre is no longer available. However at time of writing, Kumho Tyres offer a T155/90R17 'Temporary Use' tyre. This not only fits the Ford Falcon (Model FG) 17" rim, but has the advantage of a 20mm wider tread.
The Model FG Falcon has appropriate wheel offset and stud spacing and will fit the Kona EV wheel hub. Most importantly, it clears the EV's bulky rear electric parking brake mechanism.  Complete with tyre, the FG 'Temporary Spare' can be readily found at car wreckers, selling from around $50 or so.
Note: Although the FG wheel and tyre may appear unused, it has been several years since the FG Falcon was produced, so even if carried as a temporary spare, the tyre itself may need replacing.

Even though the T155/90R17 does present a challenge due to its wider tread, under-floor storage in the Kona is definitely achievable. Although the original cargo floor panel slides back into its original position, there is literally no clearance between the tyre and the underside of the floor. Obviously, the two foam storage trays have to go.

Materials Needed:
1/. Sheet of 7mm plywood (600mm X 897mm).
2/. Block of wood (about 140mm high X 350mm long).
3/. A 'Leg Plate Straight' (3/8" Whitworth thread).
4/. Piece of 3/8" threaded rod (about 100mm long).
5/. A 3/8" nut and spring washer.
6/. Large plate (wider diameter than the spare wheel centre hole), to clamp the spare in position. (I used a spare brushcutter 'wear plate'. A plywood disc would suit).
7/. A 3/8" wing nut. (I used a hammer to round over the top of the threaded rod, filed it smooth, then tightened the wing nut against it, before fitting the disc and locking the nut and spring washer up from underneath).

The mounting bolt might seem unnecessary, but it is needed to ensure the wheel stays firmly located, and clear of the exposed electrical cabling on the forward left side of the well.