The Hyper-Kamiokande (HK) experiment aims to address the most fundamental questions of particle physics today through its design as a multi-purpose water-Cherenkov detector with a 40% photocathode coverage. Proper safeguards must be taken against improbable, but catastrophic events such as a chain implosion of thousands of photomultiplier tubes (PMTs). Here, we present the evolution and testing results of the stainless-steel UAM-Aratz HK PMT cover, also known under the informal designation of "Spanish cover". The design, modelization and testing phases carried out during 2017 and early 2018 have shown it to be a reliable low-cost alternative for large-scale PMT implosion protection in HK. Follow-on work will focus on weight reduction while maintaining its robustness and economicity.