From that list, the B4.1 makes the most sense since you have a B44 already and there is a degree of parts compatability.
The B4.1 also has mid-motor conversions available if you are that way inclined (and I know a lot of people like the mid-motor cars in high-grip conditions like astro and green grass). Really good parts availability and setup knowledge.
The TRF201 is a metric copy of a B4, the parts fit and quality of Tamiya is better than Thunder-Tiger-Associated, but parts are not as easy to get, and there is only one mid-motor conversion around.
The Cougar SV runs well on high-grip, not so good on lower grip, and I have some concerns about parts quality.
The Durango looks very impressive, the best mid-or-rear-motor design yet, but until it is in the hands of normal drivers we won't know what it is really like.
A saddle will fit in the same space as a stick pack so you shouldn't have any problems.
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