First, speak to your doctor. Ages ago, in biology class, they told me that during breastfeeding, the body's natural hormonal balance is such that getting pregnant again should be unlikely. But, I'm not sure how this may be in reality.

I formerly used the Magnum XL before switching to the They Fit U22. My situation is a bit different, as my entire penis is comparable in girth to your base. However, I found that the "ring" or "bead" at the open end of the Magnum XL was quite uncomfortable. Getting the condom off was difficult, as the ring would press itself deep into my flesh. (I've never been able to just pull a condom off, I have to roll them inside out, making the ability to grip the ring with my fingers important.) When I did get the condom off, my penis would have a groove in it from the ring. The situation was vastly improved with the They Fit U22. Easy on, easy off (still have to roll it inside out), and comfortable. However, as your penis is a different shape, I'm not sure if my experience applies or not.

One problem with any standard condom made for sale in the USA is that the US Food and Drug Administration sort of requires that the condom be fairly small at some point near the base. This is out of concern of slippage or leakage on an average-width penis. Therefore, US condoms are, in general, not good for a penis with a wide flare at the base. The They Fit brand was able to circumvent the FDA width requirements by having you measure your penis, hopefully avoiding misapplication of wide condoms. However, I'm not sure the They Fit condoms (if you could even get them) will work for you, because they are not tapered, and will probably be way too loose in the middle.