  Despite their cosmological utility, the progenitors of Type Ia supernovae
(SNe Ia) are still unknown, with many efforts focused on whether accretion from
a nondegenerate companion can grow a carbon-oxygen white dwarf to near the
Chandrasekhar mass. The association of SNe Ia resembling SN 1991T ("91T-like")
with circumstellar interaction may be evidence for this "single-degenerate"
channel. However, the observed circumstellar medium (CSM) in these interacting
systems is unlike a stellar wind -- of particular interest, it is sometimes
detached from the stellar surface, residing at $\sim 10^{16}~{\rm cm}$. A
Hubble Space Telescope (HST) program to discover detached CSM around 91T-like
SNe Ia successfully discovered interaction nearly two years after explosion in
SN 2015cp (Graham et al., 2018). In this work, we present radio and X-ray
follow-up observations of SN 2015cp and analyze them in the framework of
Harris, Nugent, & Kasen (2016) to limit the properties of a constant-density
CSM shell in this system. Assuming the HST detection was shortly after the
shock crossed the CSM, we constrain the total CSM mass in this system to be $<
0.5~{\rm M_\odot}$. This limit is comparable to the CSM mass of supernova
PTF11kx, but does not rule out lower masses predicted for recurrent novae. From
lessons learned modeling PTF11kx and SN 2015cp, we suggest a strategy for
future observations of these events to increase the sample of known interacting
SNe Ia.
