This shows the identical Mallard, rather the worse for wear, fitted with a Lenz Silver Mini DCC chip placed onto a sloping card shelf.
The shelf is supported by two card uprights, superglued to the central column and to the motor support. I find that this is the neatest solution for fitting DCC chips to these larger steam locomotives.
The longer chips, or chips that require a plug in harness can not be made to fit in this location.

Lenz Silver Mini DCC chips are amongst the most expensive, and at present (Aug 2020) Bachmann have stopped producing their 36-558 small wired chip, and Gaugemaster no longer have their DCC22 Very Small Decoder'

I prefer to leave plenty of wire avaiable for modifications, so rather than connect the chip direct, I join them to other wires and use heat shrink tube on the joint. These rest happily on top of the motor casing.