The easiest is the IKEA spanking tool or the IKEA Spanish Pony.
This is the simple tressle sold by IKEA under the name FINNVARD. It’s just £25 and can be kept completely disguised either as a woodworking tressel or a table. (It does look less out of place as a table, and very useful if you ever have 2 subs playing as you can torture them both at the same time. In my case, I bought two separately and persuaded IKEA to give me spare tops, so now I have a padded one, like below, and one where the top is converted to a spanish pony. (All the tops are on view in my workshop, one marked “for non-scratch work” ( padded) and the other “for cutting work” (the spanish pony). Another Master say this and created a spanish pony with serrated upper edges (Diabolical).
(In my case, I’ve slightly re-engineered the frame to make it stronger, just to be safe, but as I usually have very small oriental for small and petite subs of 45kg and the system is rated at 85kg, it is not actually necessary.
(Finnvard £25 + £7 Conversion costs).