To install a set of saddlebags on the Royal Enfield Interceptor 650 without rails or saddlebag holders, use the saddle belts to connect two Longride saddlebags equipped with the Saddleback system.
Lift the seat and install the saddle belts on the seat sub-frame. Position the saddlebags so they rest against the suspension and the piggyback reservoirs of the Interceptor.
Put the seat back on.

This solution was suggested by a customer. The saddlebag used for this installation is the Mini Patriot SB. The saddlebags need to be mounted on their highest setting to make sure you keep a safe distance from the exhausts.
Because of their high position, the top of the saddlebags might lean forward.
The way around this is to tie them together with the nylon string that we send with the bags. See pictures below. Once the seat is put back on, it should push down on the string, forcing the saddlebags to straighten back.

The whole method can be applied to a variety of motorcycles.
Just keep in mind that the Royal Enfield Interceptor 650 has piggyback reservoirs on the suspension, which provides more support. If a motorcycle doesn't have these reservoirs on the suspensions, there is less for the saddlebags to lean on.

The interceptor has high exhausts, so you can only use smaller saddlebags.
For example, the Super Patriot would be too large. But remember you can always use the Patriot extenders to get more out of your Mini Patriot saddlebags.