A simpler option might be what eBay sellers seem to call a "half slim SSD" which should plug directly into the SATA connector without needing an adaptor, and it has no case so it's pretty small, the same or less than what you show here. SATA to SATA DOM adapters are impossible to find at the moment in the UK and the SATA DOM units themselves generally are fairly expensive here.I got it booting esxi on a satadom with that adapter and the dual 10g card in it. now i just need a back plate to hold the card.
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If you're looking at ESXi 7 then it requires 32GB minimum for an SSD and will consume up to 138GB for a boot device, so 64 or 128GB looks optimal given ESXi seems to appreciate some scratch and swap space. A 128GB half slim is £24 (~$30) delivered here at the moment.