However, having the band of own WSPR activity blocked is bad enough, even worse, having a transceiver blocked...
For 30m-WSPR, Gene (W3PM,GM4YRE) made a nice design, check this out. Nice ideas, however, I would/will do one or the other thing in a different way. Nevertheless, the basic idea is to create a DSB-signal, with some sideband separation by a simple single crystal filter. My present design ideas will not fall short of digital ICs and probably something subharmonic...
For 20m however, another nice option is available with a combination of standard computer crystals for a superhet design.
With a carrier frequency of 14,095.6kHz, the obvious choice would be a 4,096kHz and a 10,000kHz crystal. Either one could serve as I.F. in a ladder-filter. However, I found some 4,068.5kHz crystals, thus, for my attempt, 10,000kHz will be the I.F., allowing for a second frequency suiting the 20m PSK31 range.
Additional option: add yet another crystal and be ready for the 20m QRSS band, yep, that would then be a 4,000kHz crystal.
The next building project is in the queue.
QRT for now, "back to the drawing board" if you want ;-)